theorem Th5:
  emp s implies |.s.| = {}
  proof
    ex F being Function of the carrier' of X, (the carrier of X)* st
    |.s.| = F.s & (for s1 st emp s1 holds F.s1 = {}) &
    for s1, e holds F.push(e,s1) = <*e*>^(F.s1) by Def13;
    hence thesis;
  end;
